The present invention relates to fluid flow control and, more particularly, to a system and method for initializing and monitoring the operation of a servovalve system for controlling the flow of fluid within a fluid circuit.
Servovalves are often utilized to precisely manipulate or regulate the flow rate and/or pressure of fluid flowing within a fluid circuit. The fluid, which can include both liquids and gases, is typically employed to move an actuator which is conventionally comprised of a piston sealed within a cylinder. The fluid circuit moves the piston by forcing fluid into one end of the cylinder while simultaneously withdrawing or exhausting the fluid out of an opposing end of the cylinder. Servovalves are most often used in closed-loop systems wherein the position of the actuator, and velocity and/or pressure of fluid flowing within the fluid circuit is continuously monitored with a feedback device which generates system feedback signals. A controller uses the system feedback signals to generate command signals that are received by the servovalve to minimize the error between a desired position of the piston and an actual position of the piston within the cylinder.
Servovalves generally incorporate a spool which either rotates or slides axially in a housing to port the fluid flow to a desired location. Stepper motors are often utilized to move the spool relative to the housing such that the flow of fluid within the fluid circuit may be manipulated. The positioning of the spool by stepper motors is well established in the prior art. Servovalves which utilize stepper motors typically position the spool in an open-loop fashion wherein the spool must be initialized. During initialization, the spool is moved to a starting point or initialization position from where the stepper motor may initiate movement of the spool to a desired position. The initialization position may be set by a spring. The controller may command the stepper monitor to move the spool in such a manner that the controller may track a sequence of command signals from the initialization position and thus maintain a virtual spool position in its memory. As long as the stepper motor precisely tracks the sequence of command signals, the error between the desired position and the actual position of the piston may be minimized.
Unfortunately, the accuracy with which the stepper motor tracks the sequence of command signals may diminish over time due to the buildup of mechanical impediments or resistance between the spool and the housing. Such mechanical resistance may include excessive friction between the spool and the housing. The mechanical resistance may also include contamination or corrosion of the spool, housing, or other servovalve components. In the prior art, servovalves that are driven by stepper motors have been fitted with optical encoders to confirm that the actual position of the stepper motor is tracking a commanded or desired position of the stepper motor and, hence, the spool position.
In such systems, the optical encoder may generate an error signal if the stepper motor fails to track the desired position. Such error signal may be indicative of a relatively high level of mechanical resistance between the spool and the housing such that corrective action must be taken. However, without the capability to continuously monitor mechanical resistance between the spool and the housing, inaccurate positioning of the piston within the cylinder may occur such that safety may be compromised. As may be seen, it would be highly desirable to provide the capability to continuously monitor mechanical resistance between the spool and the housing such that preventative maintenance may be performed on the spool and/or housing before occurrence of a failure.
Furthermore, for prior art servovalves that utilize springs to initialize the spool, the springs are generally of a torsional type. In the case of very large servovalves requiring multiple rotations of the stepper motor to achieve the desired spool position, such springs become unwieldy arrangements of clock springs that may be impractical to implement. Prior art solutions for initializing the spool include various types of external devices such as limit switches, proximity switches, and photo sensing devices. Unfortunately, such prior art solutions present mechanical and wiring problems within the servovalve. Furthermore, such prior art solutions complicate the initialization process. As can be seen, it is highly desirable to provide an initialization means that is not dependent upon an external device.
The prior art in the field of stepper motors for monitoring of the spool and/or stepper motor position includes independent position sensors added to the servovalve to accomplish certain control functions requiring feedback. Optical encoders are one example of such independent position sensors, as was earlier mentioned. Other prior art systems for monitoring of the spool and/or stepper motor position includes commutation schemes wherein driver signals are provided in such a manner as to improve phase angle lead or lag in conventional stepper motor applications as is provided in U.S. Pat. No. 4,426,608 to Larson, et al., U.S. Pat. No. 5,256,943 to German, U.S. Pat. No. 4,949,027 to Baur, U.S. Pat. No. 4,884,016 to Aiello, and U.S. Pat. No. 4,761,598 to Lovenich.
Other prior art systems for monitoring of stepper motor position is related to improvement of dynamic positional accuracy of stepper motor control systems. For example, U.S. Pat. No. 6,013,998 to Spurr et al. (“the Spur reference”) discloses a stepper motor configured to turn a lead screw in microstepping mode. Baseline measurements of position errors occurring in the Spurr reference are utilized in an algorithm developed to reinforce sine/cosine current profiles that are typically used in microstepping a stepper motor so as to reduce overall position error. It should be noted that the position error addressed by the Spur reference is dynamic (i.e., in motion) and that the system of the Spur reference does not utilize position feedback during operation.
U.S. Pat. No. 5,029,264 to Ito et al. (“the Ito reference”) describes a recording apparatus which employs a stepper motor that is utilized in conjunction with an encoder. In the apparatus of the Ito reference, the encoder is of sufficient resolution such that its signal can be used by a controller to commutate the current to the stepper motor windings such that the stepper motor is effectively utilized as a multi-pole brushless motor. To accomplish this, the apparatus of the Ito reference synchronizes magnetic poles of the motor with pulses generated by the encoder. By commutating in this fashion, lower noise and higher speeds are obtained in dynamic operation of the apparatus. It should be noted that Ito describes an additional initialization sequence in which a carriage of the apparatus is moved so as to seek a separate photo sensor to indicate a starting position. However, the apparatus of the Ito reference does not provide a means to monitor mechanical resistance thereof.
U.S. Pat. No. 5,416,395 to Hiramatsu et al. (“the Hiramatsu reference”) describes a carriage drive control for a printer that utilizes a stepper motor in conjunction with an optical encoder. The Hiramatsu reference has two operating modes including closed-loop and stepwise. A controller switches between the two operating modes. When precise regulation of stepper motor speed is required, encoder feedback is processed by the controller and pulse-width-modulated (PWM) current signals are output to the motor in a closed-loop fashion. In both the Ito reference and the Hiramatsu reference, the action of the system is configured to regulate rotational speed of the stepper motor and the corresponding dynamic position error and does not provide a means to monitor mechanical resistance such that preventative maintenance may be performed.
Finally, U.S. Pat. No. 6,121,744 to Hoda et al. (“the Hoda reference”) discloses a control apparatus for a position control motor. As understood, the apparatus of the Hoda reference is configured to address a problem in stepper motor application that occurs when registration is lost between desired stepper motor position (as computed by a controller) and actual stepper motor position. In general, open-loop stepper motor systems in such a condition have substantial position error which is not correctable. The apparatus of the Hoda reference monitors static and dynamic error of the stepper motor position using a position feedback device. The apparatus changes the control scheme when this error exceeds ninety degrees of electrical phase difference so as to allow the apparatus to recover. However, the apparatus of the Hoda reference does not include any means to predict and loss of registration between desired and actual stepper motor position.
As can be seen, there exists a need in the art for a servovalve system that includes a simple and reliable initialization feature that does not require the addition of external components to the servovalve system. Additionally, there exists a need in the art for a servovalve system that includes the capability for detecting and monitoring mechanical resistance of a spool sliding within a housing such that preventative maintenance may be performed on the servovalve system.